The invention relates to devices for alternately winding a flexible element between first and second rotary supports to which the element is connected to form first and second rolls one of which winds up while the other unwinds, comprising a single reversible rotatable driving means actuable at will in either direction and kinematically connected simultaneously to first and second turning elements, and first and second coupling means between the first turning element and the first rotary support and the second turning element and the second rotary support respectively for driving said flexible element in either direction while maintaining it taut.
In a device of this type described in French patent specification No. 385,347 it was proposed to provide identical first and second coupling means each formed by a non-reciprocal unidirectional clutch. The turning elements were formed by toothed wheels freely mounted on shafts of the rotary supports, these toothed wheels being extended by hubs which each had a cam-forming slot cooperating with a pin fixed to the corresponding shaft to ensure driving engagement or disengagement of the turning element.
However, this known arrangement does not take into account the maximum and minimum diameters of the rolls. Now, for the flexible element, for example a band of paper, to be kept taut between the two rolls, the driven roll (i.e., that which unwinds) must be pulled by the paper at a speed of rotation which is always greater than the speed at which the driving means tends to drive it by the intermediary of a belt or clutch drive coupling. It can be shown that the relation to be satisfied is D/d .ltoreq. W2/W1 where D is the maximum diameter of the roll corresponding to the first turning element and d the minimum diameter of the roll corresponding to the second turning element, and W1 and W2 are the speeds of rotation of the corresponding turning elements.